1. Field of the Invention
The present invention is directed to a circuit which includes at least one bipolar power component as well as to a method for operating such a circuit.
2. Description of the Prior Art
Many power components have a lightly doped drift zone that defines the high-voltage strength of the power component. In order to increase the conductivity of the lightly doped drift zone in the activated condition of the power component, an emitter structure is often provided in the drift zone. The drift zone and emitter structure form a pn-junction polarized in conducting direction that is traversed by the main current. In the activated condition, charge carriers are injected into the drift zone from this pn-junction.
A specific component of this type is the insulated gate bipolar transistor (IGBT), that, for example, is known from H. Dorwish et al., Electronics Letters, vol. 21, no. 12, pp. 519-520 (1984). This type of component is a high-voltage DMOS transistor wherein the main current flows through a pn-junction polarized in the conducting direction. The pn-junction is formed by an emitter structure that is surrounded by a lightly doped drift zone having the opposite conductivity type. IGBTs can be vertically or laterally realized in a silicon substrate. Lateral IGBTs are generally used for integration in an integrated circuit.
The conductivity modulation, i.e. the increase in the conductivity of the drift zone in the activated condition due to charge carriers injected from the pn-junction, in fact improves the conductivity of the IGBT in the activated condition; however, it also leads to a number of disadvantages in comparison to an equivalent DMOS transistor. By comparison to an equivalent DMOS transistor, an IGBT exhibits reduced blockability. The Output characteristic exhibits a threshold voltage caused by the pn-junction at the emitter structure. Since the IGBT has a pnpn sequence, it tends toward latching given a high current. The term latching means a thyristor-like behavior wherein the IGBT can no longer be controlled. Due to the charge carrier injection in the activated condition, poor turn-off behavior with respect to switching time and switching losses occur given the IGBT. Another disadvantage it is not possible to integrate a free running diode in the IGBT.
Various measures have been proposed (see, for example, J. K. O. Sin, IEEE Transaction on Electronic Devices, vol. 40 pp. 1300-1305 (1993); A. R. Hefner et al., IEEE Transactions on Power Electronics, vol. PE-2, pp. 194-206 (1987); G. Miller et al., IEEE, PESC, 1989, record volume I, pp. 21-25) in order to improve the properties of an IGBT. The turn-off behavior, for example, is improved by a local shunt at the pn-junction formed by emitter structure and drift zone. A resistor path is connected parallel to the pn-junction for that purpose. As long as the main current through the IGBT remains below a critical value defined by the conductance of the shunt, it flows through the shunt. A charge carrier injection from the emitter structure into the drift zone then does not occur. When, by contrast, the main current is higher that this crucial current, then it flows through the pn-junction and through the emitter structure. In this case, the conductivity modulation of the drift zone occurs. The conducting or transmission properties of the IGBT, however, deteriorate due to the shunt.
A reduction of the emitter efficiency of the emitter structure can be achieved in the case of an n-junction drift zone by increasing the dopant concentration in front of the pn-junction formed by drift zone and emitter structure. In this case, too, the conducting or transmission properties of the IGBT deteriorate.
It is also known to reduce the emitter efficient by employing an extremely thin contact as anode to the emitter structure, however, this is also associated with a deterioration of the conducting or transmission properties.
Finally, it has been proposed to reduce the life expectancy of the charge carriers in the drift zone by doping with heavy metals such as Au or Pt or by electron bombardment. The conducting or transmission properties, however, are thereby disadvantageously influenced. Moreover, this method cannot be utilized within the framework of an IC fabrication process since it involves disadvantageous influences on other integrated components.
These problems occur not only in the case of IGBTs but also in the case of other bipolar power components such as bipolar transistors or thyristors.